Open Data supplied by Natural Environment Research Council (NERC)

Niskin Bottle

The Niskin bottle is a device used by oceanographers to collect subsurface seawater samples. It is a plastic bottle with caps and rubber seals at each end and is deployed with the caps held open, allowing free-flushing of the bottle as it moves through the water column.

Standard Niskin

The standard version of the bottle includes a plastic-coated metal spring or elastic cord running through the interior of the bottle that joins the two caps, and the caps are held open against the spring by plastic lanyards. When the bottle reaches the desired depth the lanyards are released by a pressure-actuated switch, command signal or messenger weight and the caps are forced shut and sealed, trapping the seawater sample.

Lever Action Niskin

The Lever Action Niskin Bottle differs from the standard version, in that the caps are held open during deployment by externally mounted stainless steel springs rather than an internal spring or cord. Lever Action Niskins are recommended for applications where a completely clear sample chamber is critical or for use in deep cold water.

Clean Sampling

A modified version of the standard Niskin bottle has been developed for clean sampling. This is teflon-coated and uses a latex cord to close the caps rather than a metal spring. The clean version of the Levered Action Niskin bottle is also teflon-coated and uses epoxy covered springs in place of the stainless steel springs. These bottles are specifically designed to minimise metal contamination when sampling trace metals.

Deployment

Bottles may be deployed singly clamped to a wire or in groups of up to 48 on a rosette. Standard bottles have a capacity between 1.7 and 30 L, while Lever Action bottles have a capacity between 1.7 and 12 L. Reversing thermometers may be attached to a spring-loaded disk that rotates through 180° on bottle closure.

RAPID Cruise D346 discrete nutrient sampling

Originator's data acquisition and analysis

Inorganic and total nutrients

Seawater was collected for analysis of micro-molar concentrations of dissolved nutrients; nitrate and nitrite, phosphate and silicate. Samples for inorganic nutrient analysis were collected directly into either 30 mL plastic pots or 60 mL Sterilin pots. 60 mL pots were used for collection of seawater for total nutrient analysis. The pots were rinsed with sample water at least three times before drawing the sample. When required, samples were stored in a fridge at approximately 4°C until analysis.

In general, analyses were started within 1-4 hours of sample collection using a segmented continuous-flow Skalar Sanplus autoanalyser according to standard colormetric techniques described by Kirkwood (1996). The only exception to this was the increase of the pump rate by a factor of 1.5 through the phosphate, improving the reproducibility and peak shape of the results.

5 µmol L -1 stock standard solutions prepared in Milli-Q water were used to produce working standards. Working standards were prepared in a saline solution (40 g NaCl in 1 L of Milli-Q water), which was also used as diluent for the analyses.

Total nutrients (total nitrogen (TN) and total phosphorus (TP)) were measured as nitrate and phosphate after photo-oxidation for 2 hours using a Metrohm 705 digester (Sanders and Jickells, 2002). The oxidation efficiency of the method was monitored using a Guanosine standard at two different N and P concentrations; i) 2 and 5 µmol L -1 for nitrogen, ii) 0.4 and 1 µmol L -1 for phosphorus, which produced i) 2±0.3 and 4.1±0.8 (efficiency higher than 80%) and ii) 0.2±0.3 and 0.8±0.2 (efficiency higher than 50%) respectively. The UV systems were installed inside the fume hood of the chemistry lab and a flow meter was attached in order to monitor the water flow for cooling.

At the start of the cruise all samples from all stations were UV oxidised in duplicates. However, when the two UV units started to fail, this caused a delay in the analysis process resulting in a large backlog of samples.

Ideally, the total nutrients should have be analysed together with their respective inorganic fraction in the same autoanalyser run, but the large backlog prompted a run of all inorganic nutrient samples immediately. The total nutrients were run as soon they became available upon UV oxidation (from Station 1 to Station 39). This meant analysing total nutrients in separate runs.

The number of samples being analysed for total nutrient concentrations was reduced to every third station. From station 39 and starting with station 42, one out of three casts were thus sampled for total nutrients. Whenever a Niskin bottle misfired, the available space on the UV unit rack was used for either a replicate or for the analysis of a Guanosine standard.

Once the backlog was cleared and the time between stations increased, it was decided that samples for total nutrient analysis should be taken from all casts again. However, this was not possible, due to the continuous failure of the UV systems. Repeats of whole profiles were also run for a number of stations to check the reliability of the UV digester units and accuracy of the total nutrient concentrations.

Inorganic nitrate and phosphate at nanomolar concentrations

A gas-segmented continuous-flow colorimetric method was used for both phosphate and nitrate. The chemical methods are described by Grasshoff et al., (1983). The autoanalyser is coupled with liquid waveguide capillary cells (LWCC) to achieve nanomolar levels of detection (Patey et al. 2008).

Blanks were measured with Milli-Q and low nutrients seawater (LNSW). Prior to this, the LNSW was aged for several months in the lab at room temperature and with the presence of light. Standards were measured in Milli-Q and LNSW to correct for the salt effect from the seawater matrix.

Samples were drawn from Niskin bottles on the CTD into 10% HCl clean 60 ml bottles and kept refrigerated at approximately 4 o C until analysis. Analysis was undertaken on a modified Burkard Autoanalyser with one main peristaltic pump and reaction channels, one for phosphate and one for nitrate.

The detection cells were Liquid Core Waveguide Capillary Cells (LWCC) of 2 m in length. Spectrophotometric detection was achieved using tungsten lamps as light sources and two spectrometers. These devices were linked with fiber-optic connections.

Data acquisition was conducted in two steps using Spectrasuite software. First, the spectrum of the coloured complex provided a value of the signal intensity for each wavelength. Secondly, the absorbance of the signal was measured for the wavelength of interest for each compound. The selected wavelengths for nitrate and phosphate are respectively 540 nm and 710 nm.

Some problems were encountered with the analyser. These were:

1. The software's capability to read both channels simultaneously. The software did not support the function of being given two references, one for each channel. Therefore, a reference monitor was required for the second acquisition.

2. Contamination of samples in the lab. This problem had been anticipated, so a bag, flushed with oxygen-free nitrogen, was successfully set around the sampler to prevent any contamination from the air. The first sample read was repeated at the end of the run to ensure there was no contamination.

Further details can be found in the D346 cruise report (King et al., 2012).

BODC data processing procedures

The data were supplied in MSTAR format for the nitrate, phosphate, silicate, total nitrogen, total phosphorus, dissolved organic nitrate and dissolved organic phosphorus variables and converted to ASCII. Data for nanomolar concentrations of nitrate and phosphate were supplied to BODC in a csv spreadsheet. Units were converted, if necessary, to the BODC standard parameter units. The data were then loaded into a database under the ORACLE Relational Database Management System without modification.

Data that lay outside the permitted range for the parameter code were flagged suspect with an 'M' flag by BODC.

Content of data series

Originator's parameter

Description

Units

BODC parameter code

Units

Comments

totnit

Nitrate+Nitrite

µmol kg -1

NTKGAATX

µmol kg -1

-

phos

Phosphate

µmol kg -1

PHKGAATX

µmol kg -1

-

silc

Silicate

µmol kg -1

SLKGAATX

µmol kg -1

-

tn

Total nitrogen

µmol kg -1

NTAAKGD1

µmol kg -1

-

tp

Total phosphorus

µmol kg -1

TPHSKG01

µmol kg -1

-

nitrate (nM)

Nanomolar concentrations of nitrate

nM

NTRZLWTX

µmol l -1

Unit conversion: *0.001

phosphate (nM)

Nanomolar concentrations of phosphate

nM

PHOSLWTX

µmol l -1

Unit conversion: *0.001

DON

Dissolved organic nitrogen

µmol kg -1

MDMAP008

µmol kg -1

-

DOP

Dissolved organic phosphorus

µmol kg -1

ORGPDSZZ

µmol kg -1

-

Data quality

Quality control flags that were used by the originator have been mapped to BODC flags as shown below:

Rapid Climate Change (RAPID) Programme

Rapid Climate Change (RAPID) is a £20 million, six-year (2001-2007) programme of the Natural Environment Research Council (NERC). The programme aims to improve our ability to quantify the probability and magnitude of future rapid change in climate, with a main (but not exclusive) focus on the role of the Atlantic Ocean's Thermohaline Circulation.

Scientific Objectives

To establish a pre-operational prototype system to continuously observe the strength and structure of the Atlantic Meridional Overturning Circulation (MOC).

To support long-term direct observations of water, heat, salt, and ice transports at critical locations in the northern North Atlantic, to quantify the atmospheric and other (e.g. river run-off, ice sheet discharge) forcing of these transports, and to perform process studies of ocean mixing at northern high latitudes.

To construct well-calibrated and time-resolved palaeo data records of past climate change, including error estimates, with a particular emphasis on the quantification of the timing and magnitude of rapid change at annual to centennial time-scales.

To develop and use high-resolution physical models to synthesise observational data.

To apply a hierarchy of modelling approaches to understand the processes that connect changes in ocean convection and its atmospheric forcing to the large-scale transports relevant to the modulation of climate.

To understand, using model experimentation and data (palaeo and present day), the atmosphere's response to large changes in Atlantic northward heat transport, in particular changes in storm tracks, storm frequency, storm strengths, and energy and moisture transports.

To use both instrumental and palaeo data for the quantitative testing of models' abilities to reproduce climate variability and rapid changes on annual to centennial time-scales. To explore the extent to which these data can provide direct information about the thermohaline circulation (THC) and other possible rapid changes in the climate system and their impact.

To quantify the probability and magnitude of potential future rapid climate change, and the uncertainties in these estimates.

Projects

Overall 38 projects have been funded by the RAPID programme. These include 4 which focus on Monitoring the Meridional Overturning Circulation (MOC), and 5 international projects jointly funded by the Netherlands Organisation for Scientific Research, the Research Council of Norway and NERC.

The RAPID effort to design a system to continuously monitor the strength and structure of the North Atlantic Meridional Overturning Circulation is being matched by comparative funding from the US National Science Foundation (NSF) for collaborative projects reviewed jointly with the NERC proposals. Three projects were funded by NSF.

A proportion of RAPID funding as been made available for Small and Medium Sized Enterprises (SMEs) as part of NERC's Small Business Research Initiative (SBRI). The SBRI aims to stimulate innovation in the economy by encouraging more high-tech small firms to start up or to develop new research capacities. As a result 4 projects have been funded.